a cold crash is when you drop the temperature of your brew so the yeast slows its rate of respiration and slows its rate of multiplication therefore the rate of death surpasses the rate of division so in essence you kill off your yeast without using artificial additives. You should do it if you want a more "natural" flavor and also to save money. I strongly suggest siphoning afterwards and adding irish moss to allow any additional proteins to settle out

Just thought that I would point out that it doesn't kill the yeast. It basically puts them to sleep and lets them drop out of suspension so that you end up with a clear beer. You can still harvest the yeast after you rack and save it for another batch. I like to put mine in the fridge for a couple days when fermentation is complete prior to racking to my keg.

Thanks for the heads up, I see what you mean by "putting it to sleep" I was angling from the scientific point of view that due to lower resperation you would have less divisions so when the yeast cells started dying out you would have less division to replace it.

I see where you are coming from. Over time the viability of your yeast will go down, but harvesting is a fairly common practice. There is a great sticky on how to "wash yeast" on the boards here. You can then store it in the fridge and pitch it into a starter for a new batch. A starter is essential when reusing yeast to make sure it is viable.